STATIC AND DYNAMIC DISPLACEMENTS IN RBA(2)CU(3)O(7-DELTA)(R=Y,HO, DELTA=0.05,0.5) - A NEUTRON-DIFFRACTION STUDY ON SINGLE-CRYSTALS

Mean-square displacements in superconducting RBa(2)Cu(3)O(7-delta) (R=Y, Ho) were studied as function of temperature by neutron diffraction on single crystals. From a comparison of the results with the pure dynamical displacements calculated from the phonon dispersion relations of YBa2Cu3O7 we estimate the static contributions to the displacements. In particular we find static contributions to the displacements parallel to the (a, b) plane of the chain and the apical oxygen atoms. There is no evidence of an anomalous displacement of the apical oxygen in the c direction, in contrast to extended x-ray-absorption finestructure studies suggesting a double-wed potential. We also report on a structural anomaly near T-c, which is observed in the intensity of specific Bragg reflections.